Many CPUs can be partitioned, con
tually and sometimes physically, into t
parts: the I-unit, the E-unit, and the S-u
The I-unit (instruction) is responsible
instruction fetch and decode. It may h
some local buffers for lookahead prefe
ing of instructions. The E-unit (execut
does most of what is commonly referre
as executing an instruction, and it cont
the logic for arithmetic and logical op
tions. The S-unit (storage) provides
memory interface between the I-unit
E-unit. (IBM calls the S-unit the PSCF
processor storage control function.)
The S-unit is the part of the CPU
primary interest in this paper. It cont
several parts or functions, some of w
are shown in Figure 1. The major com
nent of the S-unit is the cache mem